1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a protection circuit for a secondary battery and a secondary battery having the same, which can protect the secondary battery even in case of its being exposed to high temperature, as well as prevent the temperature of the secondary battery from increasing due to an overcharge.
2. Description of the Prior Art
Recently, as the application field of the secondary battery has been expanded to an energy source of portable terminals, camcorders, laptop PCs, Hybrid Electric Vehicles (HEVs), and Electric Vehicles (EVs), the research and the development for the secondary battery has been gradually accelerated.
Among the present secondary batteries which are available, a lithium ion secondary battery which had been developed in the early 1990's and has been in the limelight has a merit of having an operation voltage and an energy density greatly higher than those of a conventional battery including a Ni—MH battery, a Ni—Cd battery, and a sulphuric acid-lead battery, etc.
Generally, when a secondary battery such as a lithium ion battery is overcharged higher than a desired voltage, occurrence of a side reaction of a positive active material and an electrolyte increases. In a worse case, a collapse of the structure of the positive active material and an oxidation of the electrolyte, etc. may occur. Thereby, the lithium may also be precipitated from a negative active material. From such a state, if the voltage further increases, the secondary battery may explode or ignite.
Further, when the lithium ion secondary battery is exposed to a high temperature which is higher than the desired temperature, an electrode active material is unstably activated and rapidly reacts with the electrolyte so that the secondary battery expands. In several cases, the lithium ion battery may explode or ignite.
The possibilities of explosion and ignition of the secondary battery due to the exposure to high temperatures are changed according to the charge status of the secondary battery. More specifically, during the charge of the secondary battery, since the energy density in the secondary battery increases, and the instability of the electrode active material deepens, the higher the charge ratio increases, the more the possibility of an explosion or ignition increases.
In order to solve such a problem, according to the conventional art, an active device called “Positive Temperature Coefficient resistor (PTC resistor)” is connected to both electrodes in series, of which an inner resistance increases depending on the increase of the temperature of the secondary battery due to an overcharge, so as to interrupt electric current to be input and to prevent the overcharge.
However, since the PTC resistor interrupts the electric current to be input into the secondary battery only when the temperature of the secondary battery increases due to the overcharge, there is a problem in that the secondary battery cannot be protected when the already charged secondary battery is exposed to high temperatures.